This invention relates to a protection circuit for non-isolated ballasts that drive gas discharge lamps to prevent shock hazards. The device acts to prevent excessive leakage currents from flowing between the ballast and ground by introducing a higher common mode impedance path in the circuit.
In fluorescent lighting fixtures, a high voltage is required at the sockets in order to initially strike the lamp and cause an electrical arc to begin flowing between the electrodes. These high voltages pose a potential shock hazard to service people having to work on the fixtures such as during lamp replacement. To reduce this hazard, protection methods have to be employed whenever the voltages in the fixture exceed a certain level. The main shock hazard situation exists when a repairman is in good contact with earth ground. This can occur in a variety of situations such as when the repairman is holding onto a pipe for support or is working in a damp or wet environment. When one part of the person is in ground contact and is holding onto one end of a fluorescent lamp while the other end is being inserted into a lamp socket, an electrical shock situation can occur.
Two Underwriters Laboratory (UL) tests for shock hazard are required to passed by a lamp ballast to receive UL approval. The first test is called a through lamp leakage current test. See FIG. 3 for a diagram of the test set-up. This test measures the amount of current that will flow through an unstruck lamp with one end in the fixture and the other end connected to ground through a resistor. The current flow is due to the parasitic capacitance that exists between the length of the lamp and ground. Its purpose is to simulate a relamping procedure when a person is holding onto one end of the lamp and inserts the other end into an energized fixture. The second test is called a metal foil leakage current test. This test measures the amount of current that will flow between one end in the fixture and a metal foil placed around the lamp and connected to ground. A metal foil is placed around the lamp, connected to ground and is slid along the length of the lamp to test all positions along the fixture and lamp. While the foil is being moved, the maximum current is recorded. The current flow is due to the parasitic capacitance that exists between the glass wall of the lamp and ground. Its purpose is to simulate a relamping situation when a person is holding the lamp in the middle to remove or insert it into an energized fixture.
Prior attempts at reducing shock hazards have resulted in implementations with various drawbacks. The most common technique is to provide electrical isolation between the power line input, ground and the ballast output at the lamp sockets using a isolation transformer. One such implementation is illustrated by U.S. Pat. No. 4,277,726. This patent shows the isolation transformer providing electrical isolation to prevent leakage currents flowing between the fixture and ground. This solution is safe; however, it involves substantial penalties in terms of cost, size, weight and electrical efficiency. Another prior art technique is illustrated in U.S. Pat. Nos. 4,507,698 and 4,855,860. These patents show using a sensing circuit to detect ground faults or currents. When the ground current flow is detected a circuit is triggered that shuts down the inverter circuit in a ballast thus reducing the voltage at the lamp sockets to zero. This solution is workable, however; it involves substantial penalties in terms of cost, and additional circuit complexity. Another technique that could be utilized is that of a circuit interrupting socket. Unfortunately, these are a special non-standard socket that are costly and would require extensive wiring to be added to each fixture. Another possible approach is to put a switch on each fixture to turn it off during servicing. This approach would require the extra cost of including a separate switch and its associated wiring in each fixture. Also, a repairman could forget or not bother with unpowering the fixture during lamp replacement thus placing himself in a potentially hazardous situation.
A currently unmet need exists for a simple low cost technique to be incorporated into a non-isolated electronic ballast to prevent shock hazards during servicing.